1. Field of the Invention
This invention generally relates to electrochemical cells and, more particularly, to a sodium iron(II)-hexacyanoferrate(II) material, iron(II)-hexacyanoferrate(II) battery electrode, and associated fabrication processes.
2. Description of the Related Art
A battery is an electrochemical cell through which chemical energy and electric energy can be converted back and forth. Overall, the energy density of a battery is determined by its voltage and charge capacity. Lithium has the most negative potential (−3.04 V vs. H2/H+), and exhibits the highest gravimetric capacity corresponding to 3860 milliamp-hours per gram (mAh/g). Due to their high energy densities, lithium-ion batteries (LIBs) have triggered the portable electronics revolution. However both the high cost of lithium metal and the strain on natural resources render doubtful the commercialization of LIBs as large scale energy storage devices. In general, LIBs employ lithium storage compounds as the positive (cathode) and negative (anode) electrode materials. During battery cycling, lithium ions (Li+) are exchanged between the positive and negative electrodes. LIBs have been referred to as “rocking chair” batteries since the lithium ions “rock” (shuttle) back and forth between the positive and negative electrodes as the cells are charged and discharged. The positive electrode (cathode) material is conventionally a metal oxide with a layered structure, such as lithium cobalt oxide (LiCoO2), or a material having a tunneled structure, such as lithium manganese oxide (LiMn2O4), on an aluminum current collector. The negative electrode (anode) typically consists of graphitic carbon, also a layered material, on a copper current collector. During the charge-discharge process, lithium ions are inserted into, or extracted from, the interstitial spaces of the active materials.
Analogous to LIBs, metal-ion batteries employ metal-ion host compounds as their electrode materials into which metal-ions can migrate both easily and reversibly. Since Li+ has one of the smallest radii among metal ions, it is easily accommodated within the interstitial spaces of various materials including layered LiCoO2, olivine-structured LiFePO4, spinel-structured LiMn2O4, and so on. In contrast, larger metal ions such as sodium ions (Na+), potassium ions (K+), magnesium ions (Mg2+), aluminum ions (Al3+), zinc ions (Zn2+), etc., severely distort the structures of conventional Li+ intercalation materials and, consequently, destroy the host structures within several charge/discharge cycles. In light of this, new materials with larger interstitial spaces are required in order to accommodate various metal-ions for a metal-ion battery.
FIG. 1 is a diagram depicting the crystal structure of a transition metal hexacyanoferrate (TMHCF) in the form of AxM1M2(CN)6 (prior art). Transition metals are defined as elements whose atoms possess an incomplete d sub-shell or can give rise to cations (transition metal ions) with an incomplete d shell and include Groups 3 to 12 of the Periodic Table. The crystal structure of TMHCFs exhibits an open framework and is analogous to that of the ABX3 perovskite, as shown. In general, M1 and M2 are transition metal ions in an ordered arrangement on the B sites. The large, tetrahedrally coordinated A sites can host both alkali and alkaline earth ions (Ax) in addition to species such as H2O. The number of alkali (or alkaline earth ions) in the large cages of this crystallographically porous framework may vary from x=0 to x=2 depending on the respective valence(s) of M1 and M2. Conveniently, the open framework structure of the TMHCFs facilitates both rapid and reversible intercalation processes for alkali and alkaline earth ions (Ax).
Transition metal hexacyanoferrates (TMHCFs) with large interstitial spaces have been investigated as cathode materials for rechargeable lithium ion batteries,[1, 2] sodium-ion batteries,[3, 4] and potassium-ion batteries[5] By employing an aqueous electrolyte containing the appropriate alkali-ions or ammonium-ions, copper and nickel hexacyanoferrates [(Cu,Ni)-HCFs] demonstrated a robust cycling life with 83% capacity retention after 40,000 cycles at a charge/discharge current rate of 170C.[6-8] In spite of this, the materials demonstrated low capacities and energy densities due to the facts that (1) only one sodium-ion (Na+) could be inserted/extracted into/from per Cu-HCF or Ni-HCF formula, and (2) the TM-HCFs electrodes were restricted to operation below 1.23 volts (V) due to the electrochemical window for water decomposition. In order to compensate for such shortcomings, manganese hexacyanoferrate (Mn-HCF) and iron hexacyanoferrate (Fe-HCF) were employed as cathode materials in non-aqueous electrolyte systems.[9, 10] When assembled into batteries with sodium-metal anode, Mn-HCF and Fe-HCF electrodes delivered capacities of ˜110 mAh/g when cycled between 2.0 and 4.2 V.
It is worth noting that it is extremely difficult to directly obtain Na2Fe2(CN)6 through a conventional precipitation method. Typically, upon addition of an Fe2+-containing solution into a solution of Fe(CN)64−, Fe2+-ions are immediately oxidized to afford a blue precipitate of Na1-xFe2(CN)6. Electrochemical methods have been used to determine that X equals 0.48 for a Na1-xFe2(CN)6 sample synthesized in this manner. Furthermore, the small Na+ content confirms that a certain proportion of Fe(II) in Fe(CN)64− was similarly oxidized during the process. In 2011, Hu et al. reported a hydrothermal method to synthesize K2Fe2(CN)6 from K4Fe(CN)6 in a neutral pH solution.[11] However, it is difficult to synthesize Na2Fe2(CN)6 due to the fact that sodium ions are smaller than potassium ions and are therefore harder to retain in the large interstitial space of Fe-HCF. In addition, the reaction is sensitive to the pH of the reaction solution.[12-14] In acidic solutions (pH<7), K4Fe(CN)6 produced a Prussian blue material, namely KFe2(CN)6, using a hydrothermal reaction process. In alkaline solutions (pH>7), K4Fe(CN)6 decomposed and formed iron (II, III) oxide (Fe3O4). In light of these results, it may be surmised that the formation of Na2Fe2(CN)6requires specific solution pH unlike the neutral conditions that were found to be satisfactory for synthesizing K2Fe2(CN)6. However, the deliberate adjustment of the pH for reaction solutions also failed to produce Na2Fe2(CN)6.
It would be advantageous if a process existed that was able to directly synthesize Na1+XFe[Fe(CN)6], where X is less than or equal to 1.
[1] V. D. Neff, “Some Performance Characteristics of a Prussian Blue Battery”, Journal of Electrochemical Society 1985, 132, 1382-1384.
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[3] Y. Lu, L. Wang, J. Cheng, and J. B. Goodenough, “Prussian Blue: a New Framework for Sodium Batteries”, Chemistry Communications 2012, 48, 6544-6546.
[4] L. Wang, Y. Lu, J. Liu, M. Xu, J. Cheng, D. Zhang, and J. B. Goodenough, “A Superior Low-Cost Cathode for a Na-ion Battery”, Angewandte Chemie International Edition 2013, 52, 1964-1967.
[5] A. Eftekhari, “Potassium Secondary Cell Based on Prussian Blue Cathode”, Journal of Power Sources 2004, 126, 221-228.
[6] C. D. Wessells, R. A. Huggins, and Y. Cui, “Copper Hexacyanoferrate Battery Electrodes with Long Cycle Life and High Power”, Nature Communications 2011, 2, Article number: 550.
[7] C. D. Wessells, S. V. Peddada, R. A. Huggins, and Y. Cui, “Nickel Hexacyanoferrate Nanoparticle Electrodes for Aqueous Sodium and Potassium Ion Batteries”, Nano Letters 2011, 11, 5421-5425.
[8] C. D. Wessells, S. V. Peddada, M. T. McDowell, R. A. Huggins, and Y. Cui, “The Effect of Insertion Species on Nanostructured Open Framework Hexacyanoferrate Battery Electrodes”, Journal of the Electrochemical Society 2012, 159, A98-A103.
[9] T. Matsuda, M. Takachi, and Y. Moritomo, “A Sodium Manganese Ferrocyanide Thin Film for Na-ion Batteries”, Chemical Communications 2013, 49, 2750-2752.
[10] S-H. Yu, M. Shokouhimehr, T. Hyeon, and Y-E. Sung, “Iron Hexacyanoferrate Nanoparticles as Cathode Materials for Lithium and Sodium Rechargeable Batteries”, ECS Electrochemistry Letters 2013, 2, A39-A41.
[11] M. Hu and J. S. Jiang, “Facile Synthesis of Air-Stable Prussian White Microcubes via a Hydrothermal Method”, Materials Research Bulletin 2011, 46, 702-707.
[12] S-H. Lee and Y-D. Huh, “Preferential Evolution of Prussian Blue's Morphology from Cube to Hexapod”, The Bulletin of the Korean Chemical Society 2012, 33, 1078-1080.
[13] M. Hu, Jiang, C-C. Lin, and Y. Zeng, “Prussian Blue Mesocrystals: an Example of Self-Construction”, CrystEngComm 2010, 12, 2679-2683.
[14] M. Hu, R-P. Ji, and J-S. Jiang, “Hydrothermal Synthesis of Magnetite Crystals: from Sheet to Pseudo-Octahedron”, Materials Research Bulletin 2010, 45, 1811-1715.